Helical compression springs are machine elements required in large numbers and different configurations in numerous application areas. Helical compression springs are, for example, required in large quantities as supporting springs or valve springs in automobile construction. A helical compression spring may be described as a wound or coiled compression spring of wire with spacings between the turns.
Of particular importance for the reliable functioning of helical compression springs during use as intended are the spring ends, i.e. the two axial end regions of the helical compression springs. The spring ends transfer spring force to the connecting bodies and should generally be formed such that they are compressed as axially as possible in every position of the spring. Spring end grinding, i.e. machining material away from the spring ends by grinding, plays a part in this respect in creating on the spring ends bearing surfaces for the connecting bodies that are sufficiently at right angles to the spring axis.
Spring end grinding is part of the sequence of processes involved in the production of a helical compression spring from cold-formed wire. This sequence of processes comprises many further production steps, which finally lead to a ready-to-fit helical compression spring. Cost-effective production of helical compression springs is only possible if efficient production processes are realized in the various stages of the process. Spring end grinding is of particular importance in this respect since a large part of the production costs arising in helical compression springs are incurred by this operation. Therefore, considerable efforts are undertaken to control the process of spring end grinding such that the helical compression springs can be produced with high productivity without impairing the quality of the finished products.
In many areas, a double-sided face grinding process with unstressed springs has become the established method of spring end grinding. As is known, grinding with a rotating tool is a machining process involving geometrically undefined cutting. The term double-sided face grinding process is based on the type of surfaces to be generated (flat faces), the number of surfaces to be ground (two), the part of the grinding wheel primarily in engagement (side surface) and the process (grinding). A particular feature of that process is the fact that the helical compression springs apply the grinding pressure themselves.
A numerically controlled spring end grinding machine suitable for the double-sided face grinding process has a grinding unit, a loading unit and a control unit that controls the loading unit and the grinding unit. The grinding unit has a pair of grinding wheels comprising two rotatable grinding wheels, the axes of rotation of which are normally arranged coaxially in relation to one another or slightly tilted with respect to one another. Formed between the mutually facing side surfaces of the grinding wheels is a grinding space. The loading unit has at least one loading plate rotatable more or less axially parallel to the grinding wheels and has a plurality of out-of-axis spring receptacles, each to receive a helical spring. The spring axes of the helical compression springs received in the spring receptacles should in this case be as parallel as possible to the axis of rotation of the loading unit and, consequently, perpendicular to the grinding side surfaces of the grinding wheels.
In the grinding operation, there is a distance between the axes of the grinding wheels and the axis of rotation of the loading plate. During a grinding operation, those helical compression springs that have been received in spring receptacles of the loading plate are successively transported through the grinding space between the rotating grinding wheels by rotation of the loading plate. As this happens, both spring ends of the helical compression springs located in the grinding space are simultaneously machined by grinding.
The distance between the center of rotation of the loading plate and the center of the grinding wheels thereby determines the position of the grinding path. The grinding path or trace describes the path over the grinding wheel that the helical compression springs cover during rotation of the loading plate. The trace, the grinding rate, the rotational speed of the loading plate and the grinding pressure together determine the achievable grinding performance.
With regard to high productivity, it is generally endeavored to achieve a grinding performance that is as high as possible, i.e. a rate of removal per unit of time that is as high as possible. However, grinding performance is restricted by admissible temperature of the spring material and capability of the grinding wheels. If the spring material becomes too hot, material displacements may occur, adversely influencing the later behavior of the spring and/or the strength of the material. Therefore, material overheating should be avoided as much as possible.
Some processes provide active cooling of the grinding space and/or the loading plate. In the case of grinding space cooling, for example, fresh air is blown directly into the grinding space, with the aim of cooling down the helical compression springs, the chips and the abrasive particles, dissipating the frictional heat and blowing out the chip spaces. In the case of loading plate cooling, cooling air is blown into the helical compression springs. The goal is to increase the specific removal capacity by keeping the temperature of the helical compression springs in the process as constant as possible and sufficiently low.
JP 2009-279709 A describes a spring end grinding machine for double-sided face grinding in which two cooling plates parallel to one another are provided directly next to the grinding wheels outside the grinding space and their mutually facing end faces lie substantially as an extension of the mutually facing side surfaces of the grinding wheels. The cooling plates, which are cooled by a cooling fluid passed therethrough, bound a space which lies between the cooling plates and in which the helical compression springs move during rotation of the loading plate as soon as they leave the grinding space. The spring ends are in this case in physical contact with the cooling plates. In this way, contact cooling of the spring ends during a grinding operation is possible.
It could therefore be helpful to provide a method of grinding spring ends of helical compression springs and also a spring end grinding machine that carries out the method that can work with high productivity and at the same time offer a high degree of certainty in safeguarding against overheating of the machined helical compression springs.